1. Field of the Invention
The present invention relates to an ink containing nanoparticles for formation of thin film of a solar cell and its preparation method, CIGS thin film solar cell (abbrev. to “CIGS solar cell”) and a method for manufacturing the same by using the ink. More particularly, the present invention relates to CIGS thin film solar cells and a method of manufacturing the same, comprising formation of a CIGS thin film by only a printing process of an ink containing nanoparticles without requirement of vacuum processing or complex equipment, characterized in that it can freely adjust Cu/(In+Ga) ratio and/or Ga/(In+Ga) ratio of the CIGS thin film and form the CIGS thin film through a simple coating or printing process.
2. Description of the Related Art
A solar cell is a device to convert light energy into electric energy by a photo-voltaic effect. The solar cell is generally classified into silicon solar cell, thin film solar cell, dye sensitive solar cell and organic polymer solar cell on the basis of constitutional substances of the cell.
Such a solar cell is independently employed as a main power source for electronic clock, radio set, un-manned light house, satellite, rocket, etc. and is connected to a regular AC power supply system to serve as an auxiliary power source. As a demand for alternative energy grows, an interest in solar cells is increasing.
For the solar cell, it is very important to increase conversion efficiency related to a conversion rate of incident sunlight into electrical energy. There are a number of studies and examinations to increase the conversion efficiency. Also, Technical developments are actively underway, which include an art of applying a thin film with high light absorption coefficient to the solar cell in order to improve the conversion efficiency.
As one of materials with high light absorption coefficient, CIGS (copper indium gallium selenide) was proposed to enhance light absorption coefficient of a thin film solar cell when using CIGS to produce the thin film solar cell.
In recent years, there have been suggested methods for formation of CIGS films for CIGS solar cells including, for example: co-evaporation method of Cu, In, Ga and Se; selenization method under Se or H2Se atmosphere after formation of Cu, In and Ga films by means of sputtering or vapor deposition; and the like.
Methods for formation of CIGS thin films using printing processes are now proposed, which do not need an expensive vacuum process and are, thus, advantageous to reduce production costs in manufacturing solar cells.
However, among the printing processes, a method for reduction and selenization of copper indium oxide film using a precursor has disadvantages in that it necessarily demands the reduction stage and uses H2Se, known to be a toxic gas, therefore, is not preferable and/or has serious problems in its application to manufacturing CIGS solar cells.
In order to reach the highest conversion efficiency, CIGS preferably has a band gap ranging from 1.2 to 1.4 eV. The band gap can be controlled by altering Ga doping concentrations and, in order to obtain the best band gap energy, the doping process should be carried out with the compositional ratio of Ga/(In+Ga) (hereinafter, abbrev. to “Ga/(In+Ga) ratio”) ranging from 0.3 to 0.6. However, in case of using a Copper Indium Oxide thin film, Ga is precipitated therein and it is difficult to dope the thin film at a constant ratio therewith.
In regard to the CIGs thin film, if Cu/(In+Ga) ratio is less than 1, a Cu-poor single chalcopyrite phase is generated which has poor performance due to a small grain size. On the other hand, when Cu/(In+Ga) ratio is more than 1, the grain size is increased which results in the performance being more improved, but, in a Cu-rich phase, there are disadvantages in that Cu2Se impurities are generated and derive a decrease in the light conversion efficiency caused by higher conductivity of Cu2Se.
In order to accomplish advantages in both of the cases that Cu/(In+Ga) ratio is more than or less than 1, there is required a multi-stage process for regulating a concentration of Cu/(In+Ga) step by step.
Accordingly, there is a requirement for techniques to form CIGS thin films of CIGS solar cells, which can form CIGS thin films without requirement of expensive vacuum processes and/or complex equipment, freely regulate Cu/(In+Ga) ratio and Ga/(In+Ga) ratio and alter concentration of Cu/(In+Ga) step by step through the multi-stage process.